Released membranes are usually used as moving parts in MEMS. The membranes are formed on sacrificial layers. When the sacrificial layers are etched away, membranes are released. The released membranes can move by deflection, and they often act as functional parts in MEMS (micro electro mechanic system). For example, they can have high reflectivity, acting as mirrors. In some applications of MEMS, such as optical filter, released membranes need some kinds of profiles, such as convex or concave surfaces, instead flat ones to perform well. The deflection by outside force will change the profile of released membranes, but the curvatures vary with deflection and usually the curvatures radii are very large. Compressive stress can be introduced on the membranes, leading a bow profile of membranes. Compressive stress will make membranes not easy to be released and the curvatures will vary with deflection. This method can not make surfaces with small curvature radii.
U.S. Pat. No. 6,768,756 gives a method to make curved membranes. In that patent, patterned photo resist is reflowed to make curved surface and this curved surface is transferred to a device layer by etching and membranes having optical properties are coated on the device layer. The device layer must be thick enough for forming curved surfaces. The membrane is attached to the device layer. Therefore, very large driving force is needed to move membranes. And some more optical membranes will be needed to eliminate the effect of the device layers if light pass through them.